React is a view-centric component system. It's primarily used for view-related stuff, but the component abstraction is pretty powerful, so they can be useful in lightweight cases for application structure too (YMMV here).

React Native is a system of bindings of native APIs to Javascript. This includes bindings which allow you to use native UI elements just like you would use divs, inputs etc. for React on the web, but it also includes bindings for common mobile OS APIs like geolocation, camera, maps, etc.